The present invention relates generally to television receiver sound systems and, more particularly, to a high-fidelity intercarrier signal detection circuit for developing a relatively clean 4.5 MHz intercarrier sound signal to facilitate the processing of a wideband television sound signal such as a stereophonic sound signal.
A number of proposals have been made over the years for the transmission of a stereophonic sound signal together with a conventional NTSC television picture broadcast. In order to make the transmission of a conventional television picture signal with a stereophonic sound component commercially attractive, it has been recognized that significant improvements in the sound reproducing qualities of conventional television receivers are required. In particular, the well known phenomenon of intercarrier buzz characterizing current television receiver sound systems becomes highly objectionable in the case of a stereophonic sound signal due to its increased bandwidth.
It is well known that an audible buzz, commonly referred to as intercarrier buzz, is produced by conventional television receiver sound systems. While not being especially objectionable in present monaural television receiver sound systems, the intercarrier buzz phenomenon imposes a severe limitation on proposed high-fidelity stereophonic sound systems for television receivers. Sources of intercarrier buzz in a television receiver generally result from various non-linearities characterizing the receiver's sound signal processing path, one such source being the tilt or Nyquist slope centered about the 45.75 MHz picture carrier of the receiver's intermediate frequency response characteristic. This tilt or slope, which is necessary to compensate for the vestigial sideband nature of the picture signal, imposes unequal gain on the modulation components of the picture signal on opposite sides of the 45.75 MHz picture carrier thereby phase modulating the carrier signal. When applied to a video detector, the phase modulated picture carrier will beat with the intermediate frequency sound signal to produce a 4.5 MHz intercarrier sound signal which, in addition to the desired audio frequency modulation, will also exhibit the undesired phase modulation of the picture carrier resulting in spurious buzz components. The effect of these spurious buzz components is more objectionable in a stereophonic sound system as compared to a monaural sound system since more of the spurious picture carrier sidebands are demodulated with a larger bandwidth stereophonic sound signal.
It is therefore a basic object of the present invention to provide an improved high-fidelity television receiver sound system.
It is a more specific object of the invention to provide an improved high-fidelity intercarrier signal detection circuit for a television receiver sound system which is adapted for producing a relatively clean 4.5 MHz intercarrier sound signal.
It is yet another object of the invention to provide an improved high-fidelity intercarrier signal detection circuit for a television receiver sound system which is adapted for reducing intercarrier buzz to facilitate the processing of a wideband television sound signal such as a stereophonic sound signal.
It is a still further object of the invention to provide an improved intercarrier signal detection circuit for a television receiver which is used to facilitate the production of an enhanced quality detected video signal.